It’s funny, but what I use to check the level is a piece of underwear seam with a bullet zip-tied into the end of it.
(I am certain there are alternative approaches)
But what you do is have about a 12 inch piece of flexible fabric with a weight on it. You mark a line on it so you know how far to lower it (you align this line with the TOP of the overflow tank) and then, when you reel it up, the wetness level is the fluid level inside your overflow tank.
PS… You don’t really need a catch tank, because once your system reaches equilibrium, it will stop peeing fluid out. At that point you have set it, and you can forget it….
Except all of us conscientious Pantera owners make sure that we have a good fluid levels, so we will check it every outing!